Synthetic resins, including polypropylene, polycarbonate, polystyrene, polyesters and aliphatic polyamides, such as nylon-6,6 and nylon-6, and copolymers and blends thereof, are often used for molded articles and fibers. In many uses, particularly for molded articles, it is preferred that the resin have improved resistance to fire, compared to the resin alone. This is often attained by the addition of various agents which improve the basic fire resistance of the resin, but sometimes these agents degrade or diminish other physical properties of the resin, such as electrical resistance, tensile strength, and elongation. Since resins are widely used, compositions which have improved fire resistance but yet retain the desirable physical properties of the resin are constantly being sought.
German Patents 2,150,484 and 2,130,793, and A. E. Lipska, Comb. Inst. Arizona State Univ., West. State Sect. Combust. Inst. WSCI, 1973, report that certain tungsten compounds can be used in various ways to improve the fire resistance of polyamides. The combinations of agents described hereinafter are not disclosed in these references.
U.S. Pat. No. 4,298,518 discloses compositions containing polyamides and melamine cyanurate, which are said to have good fire resistance.
U.S. Pat. No. 3,458,470 discloses compositions containing polyamides and a variety of tungsten or molybdenum compounds, including silico- and phospho-tungstic acids. These compositions are said to have good resistance to discoloration and be stable to light. U.S. Pat. No. 3,936,416 discloses that polystyrene (PS), polypropylene (PP), and polyethylene (PE) are flame proofed by combination of melamine pyrophosphate (MPP) and dipenterythritol, but requires a 45% loading of the total resin plus flame retardant composition.
Melamine phosphate may be added to synthetic resins to improve the flame retardancy of the resins, but when heated to normal engineering polymer processing temperatures the melamine phosphate reacts so that the resultant mixture of the resin and the melamine phosphate has very poor physical properties.
U.S. Pat. No. 5,618,865 discloses compositions containing polyesters or polyamides, reinforcing agents, and certain flame retardant compositions of melamine phosphate or melamine pyrophosphate; however these compositions have poor electrical properties. An important measure of the electrical properties of a resin composition is the Comparative Tracking Index (CTI). When a voltage is applied across the surface of compositions such as those in U.S. Pat. No. 5,618,865, the samples can catch fire and cause tracking as measured by CTI. Although the melamine phosphates add flame resistance, they degrade CTI unacceptably. In U.S. Pat. No. 5,708,065, compositions of polyesters or polyamides and zinc borate are used with melamine phosphates to improve CTI to an acceptable value. However, processing such compositions is difficult and flame retardancy is reduced. Samples with zinc borate made according to U.S. Pat. No. 5,708,065 have shown unacceptable mechanical properties and attempts to prepare such compositions on commercial scale large machines has proved elusive.
What are needed, therefore, are flame retardant resin compositions with acceptable electrical properties which do not have the problems and deficiencies of the prior art, and particularly compositions with mechanical properties that are not significantly reduced by the addition of flame retardants and other desirable additives.